The invention relates to an interferometer module adapted for measuring a displacement between a reference mirror, for instance provided on the optical column of an exposure tool, and a measurement mirror, for instance provided on a target carrier of the exposure tool which is moveable relative to the optical column. The invention further relates to lithography system comprising such an interferometer module, and a method for measuring such a displacement.
U.S. Pat. No. 7,224,466 provides a compact differential interferometer for measuring a displacement between a measurement mirror and a reference mirror along two axes. The interferometer uses shared measurement and reference beams that respectively reflect from measurement and reference mirrors before that shared beams are split into individual beams corresponding to the measurement axes of the interferometer. By essentially reflecting the measurement beam and reference beam twice in the respective measurement mirror and reference mirror, the beam path is extended and the resolution of the interferometer improved. A drawback of the known interferometer module is that it not possible using said module to unambiguously determine a direction of a displacement between said measurement mirror and said reference mirror, i.e. whether they are brought closer together or farther apart.
It is an object of the present invention to provide a interferometer module allowing determination of a direction of displacement between a measurement mirror and a reference mirror.